Until recently, bicycle frames have been constructed of metal tubes joined at their ends as by welding, or are brazed or soldered onto metal lugs, forming the frame. Recently, composite materials have been utilized in place of metal for frame construction. Such composite materials have a lower density, higher specific strength and stiffness, and better damping qualities than traditional metals, and thereby provide an increase in frame strength and stiffness with a reduction in weight, as compared to earlier metallic frames.
In construction of bicycle frames using such composite materials, the most common method for joining frame tubes, formed from such composite materials to each other, has consisted of adhesively joining the tubes to metal lugs or joint structures, thereby forming the bicycle frame. The disadvantage of using metallic lugs is their weight relative to composites, the weight of which metallic lugs significantly exceeds the weight of the frame composite tubes, thereby greatly limiting potential weight reductions. Also, the relatively high material density of metallic lugs has tended to favor a use of smaller diameter tubes and hence smaller lugs for weight savings, constituting a lesser strength frame with diminished damping qualities than would be provided utilizing larger tubes. The material density characteristics of metallic lugs has also prevented the development of structurally efficient large gussetted aerodynamic shapes for the lugs on account of the weight increase inherent with such shapes.
Where manufacture of one-piece all-composite unitary bicycle frames has been implemented, such manufacture has been found, for one thing, to be labor intensive. Further, the use of lower than optimum compaction pressures in actual practice has reduced material strength, and has required a greater use of materials than anticipated, resulting in a greater frame weight than predicted. Due to a frequency of structural strength problems and structurally related cosmetic problems occurring in service, such frames have been found to be generally unreliable.